Columnar-grained polycrystalline solar cell substrate

ABSTRACT

The invention relates to a silicon sheet having columnar grains extending axially through the sheet from one free surface of the sheet to the other free surface. The sheet has an electrical resistivity in the range of 0.1 to 10 ohm-cm.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 09/033,155, filedMar. 2, 1998, which in turn is based upon provisional application SerialNo. 60/039,418, filed Mar. 4, 1997.

BACKGROUND OF THE INVENTION

Photovoltaic solar cells are semiconductor devices which convertsunlight into electricity. Solar cells based on crystalline siliconoffer the advantage of high performance and stability. The principalbarrier to expanded utilization of silicon solar cells for electricpower generation is the present high cost of the solar cells.

In conventional solar cells based on single crystal or large grainpolycrystalline silicon ingot processes, the major cost factor isdetermined by the requirement of sawing ingots into wafers. Sawing is anexpensive processing step, and furthermore results in the loss ofapproximately half the costly ingot material as silicon dust. Theproblem to be solved requires the development of a low-cost process,that efficiently employs low-cost materials while maintaining solar cellperformance.

The solution to the problem requires the achievement of a process thatis controllable, has high a real throughput, and generates material withadequate crystalline morphology. Prior art includes several processeswhich either effectively achieve controlled growth, or high a realthroughput of silicon sheet or ribbons. All these approaches eliminatethe costly process of sawing large areas to create wafers from ingots.For example, publications by Hopkins (WEB), Ettouney, et al. (EPG),Gutler (RTR) and Eyer, et al. (SSP) describe processes that achievecontrolled polycrystalline growth of grains greater than 1 mm in size atlow linear speeds (and consequently low a real generation rates). Commonto these sheet manufacturing processes is the fact that the sheetpulling direction and the direction of grain growth are collinear. Allof these processes employ a large temperature gradient (>500 degreesCentigrade per centimeter) along the sheet pulling direction. Thisgradient is necessary to achieve the practical linear sheet pullingvelocity (typically less than 2 cm/min), but also introduces largethermal-induced stresses. In many cases these stresses limit theachievable practical sheet width by causing sheet deformations whichmake solar cell fabrication untenable. Thermal stresses can also createcrystalline defects which limit solar cell performance. Each of theseprocesses attempts to achieve grain sizes that are as large as possiblein order to avoid the deleterious effects of grain boundaries on solarcell performance.

Another set of processes has been developed that can achieve high a realthroughput rates. For example, publications by Bates, et al. (LASS),Helmreich, et al. (RAFT), Falckenberg, et al. (S-Web), Hide, et al.(CRP) Lange, et al. (RGS) and Hall et al. (SF) describe processes thatachieve polycrystalline sheet growth with grain sizes in the 10 micronsto 3 mm range at high linear rates (10 to 1800 cm/min). Typically, theseprocesses have difficulty maintaining geometric control (width andthickness) (e.g. LASS, RAFT, RGS), and/or experience difficulty withcontamination of the silicon by the contacting materials (e.g. RAFT,S-Web, CRP). The process of Hall, et al. (U.S. Pat. Nos. 5,336,335 and5,496,416) effects geometric control and minimizes the contact of thegrowing silicon with deleterious materials. Common to these sheet growthprocesses is the fact that the sheet pulling direction and the directionof crystalline grain growth are nearly perpendicular. It is thiscritical feature of these processes that allows the simultaneousachievement of high linear sheet pulling velocities and reduced crystalgrowth velocities. Reduced crystal growth velocities are necessary forthe achievement of materials with high crystalline quality.

The prior art regarding the fabrication of solar cells frompolycrystalline silicon materials requires that the grain size begreater than 1.0 mm. This requirement on grain size was necessitated bythe need to minimize the deleterious effects of grain boundaries evidentin prior art materials. Historically, small-grained polycrystallinesilicon (grain size less than 1.0 mm) has not been a candidate forphotovoltaic material due to grain boundary effects. Grain boundaryrecombination led to degradation of voltage, current and fill factors inthe solar cell. Previous models, for example Ghosh (1980) and Fossum(1980), based on recombination at active grain boundaries correctlypredicted performance of historical materials. These models teach thatif active grain boundaries are present, they prohibit the utilization ofsmall grained materials in high performance solar cells.

SUMMARY OF THE INVENTION

It is the object of this invention to provide a low-cost process forforming low stress, columnar-grained sheets that are employed in highperformance solar cells.

A further object of this invention is to provide techniques formanufacturing columnar-grained polycrystalline silicon sheets for use asa substrate in solar cells, which overcomes the disadvantages of theprior art.

A yet further object of this invention is to provide a process formanufacturing a low-cost solar cell that employs small-grainedpolycrystalline silicon with low-activity grain boundaries.

A still further object of this invention is to provide a substrate and asolar cell made from such process.

In accordance with this invention the sheet is formed by using acolumnar growth technique that manages the details of nucleation,growth, and heat flow to control the material quality and decouple thegrain growth velocity from the linear sheet pulling velocity of thepolycrystalline material. The process begins with granular silicon thatis applied to a setter material; the setter and silicon are thensubjected to a designed thermal sequence which results in the formationof a columnar-grained polycrystalline silicon sheet at high a realthroughput rates. The equipment employed to accomplish the processincludes a distributed source of energy application, such as bygraphite-based infrared heating. The invention may also be practicedwith a process which includes a line source, such as by opticalfocusing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a perspective view showing the sequence forfabricating low stress, columnar-grained silicon sheets usable as solarcells substrates;

FIG. 2 illustrates a perspective view showing the sequence of nucleationand growth of the silicon sheet in accordance with this invention; and

FIG. 3 illustrates the manifold of process steps leading to themanufacture of the same.

DETAILED DESCRIPTION

The present invention is directed to techniques used for making improvedcolumnar grain polycrystalline sheets which are particularly adaptablefor use as substrates or wafers in solar cells. The techniques are basedupon the techniques in U.S. Pat. Nos. 5,336,335 and 5,496,446, all ofthe details of which are incorporated herein by reference thereto. Theability to use the sheet as a solar cell substrate makes possible theprovision of a solar cell consisting entirely of silicon material wherethe sheet would function as a substrate made of silicon and theremaining layers of the solar cell would also be made of silicon. Thedesired properties of the columnar-grained silicon sheet or substratefabricated with the teaching of this invention are: flatness, a smoothsurface, minority carrier diffusion length greater than 40 microns,minimum grain dimension at least two times the minority carrierdiffusion length, low residual stress and relatively inactive grainboundaries. Since the minimum grain dimension of the columnar grainsilicon sheet is at least two times the minority carrier diffusionlength which in turn is greater than 40 microns, the columnar grainswould have a grain size greater than 80 microns. Grain sizes down to 10microns can be employed with minority carrier diffusion length greaterthan 10 microns, and will lead to solar cells having lower currents, andlower power. The desired properties of a process for fabricatingcolumnar-grain silicon material appropriate for inclusion in a low-costsolar cell in accordance with the teachings of this invention are: lowthermal stress procedure, controlled nucleation, high a real throughput,and simple process control.

The criteria for the columnar-grain silicon material product of flatnessand smoothness are required to make solar cell fabrication tenable. Therequirements on diffusion length and grain size are to minimizerecombination losses in the bulk and at grain surfaces (i.e. grainboundaries), respectively. The requirement of relatively inactive grainboundaries is to effect the minimization of grain boundaryrecombination. The requirement of low residual stress is to minimizemechanical breakage and to maintain high minority carrier diffusionlengths.

The criteria for the columnar-grained silicon process of a low thermalstress procedure is to effect minimization of bulk crystalline defects.The requirement of controlled nucleation is to affect the achievement ofthe required grain morphology and size. The criteria for high a realthroughput and simple process control are to achieve low-cost andmanufacturability.

FIG. 1 is a perspective view illustrating the sequence for fabricatinglow stress, columnar-grained silicon sheets. The process as depictedmoves from left to right. In general, a setter material 100, whichserves as a mechanical support, is coated with a granular silicon layer200, and is passed through a prescribed thermal profile. The prescribedthermal profile first creates a melt region 300 at the top of thegranular silicon 200, and then creates a nucleation and growth region400 where both liquid and a growing layer of polycrystalline layercoexist. Finally, there is an annealing region 500 where the temperatureof the polycrystalline silicon sheet layer 600 is reduced in aprescribed manner to effect stress relief. Any or all of the preheat,melting, growth and anneal thermal profiles for the granular powder andresultant sheet could be achieved by graphite-based heater technology.

The setter material 100 is selected based on the following requirements.It must: maintain its shape during the sheet formation thermalprocessing; not chemically interact with, or adhere to, the siliconmaterial; and possess the proper thermal characteristics to effect therequired sheet growth and annealing. The form of the setter material mayeither be as a rigid board or as a flexible thin belt.

Several materials including, but not limited to, quartz, refractoryboards (e.g. silica and/or alumina), graphite, silicon nitride andsilicon carbide have been employed and maintained the proper geometricshape during thermal processing.

To assure that the setter 100 does not adhere to the finalpolycrystalline silicon sheet 600, a release agent coating 110 isapplied to the setter. Either, or a combination of, silicon nitride,silicon oxynitride, silica, powdered silicon, alumina, silicon carbideor carbon in any form have been employed as this agent. A low-costmethod for applying this coating is to form a liquid slurry that ispainted or sprayed on the bare setter, and subsequently dried beforeuse. The release agent coating may also be applied by the method ofchemical vapor deposition. The release agent facilitates separation ofthe sheet and permits reuse of the setter material.

In the process design the thermal characteristics of the setter 100 playa key role in managing the melt and growth processes. In the melt region300 it is preferred that the thermal conductivity of the setter be highto assure the efficient deployment of the energy being used to melt thegranular silicon 200.

In a preferred embodiment the setter material is graphite. The setterpreparation is completed by coating the top surface with a release agent110. This is accomplished using an aqueous colloidal solution of siliconnitride that is painted on the top surface and baked to form anon-wetting, non-adhering oxynitride layer, before the initialapplication of granular silicon.

The granular silicon 200 must be properly sized and be of adequatepurity. The range of proper sizes for the granular silicon 200 employedin the process is between 20 and 1000 micrometers. The upper limit isdetermined by the design thickness for the silicon sheet material.Preferably the silicon powder is comprised of sizes less than 500microns and the sheet is formed at a sheet pulling of greater than 20cm/min. As a rule the dimensions of the silicon particles should beequal to or less than the desired thickness of the silicon sheetmaterial. The lower size limit of the particle distribution is dependenton the dynamics of the melting process, and the need to limit the amountof silicon oxide. The silicon oxide is a source of sheet contamination,and naturally occurs at all silicon surfaces.

There are several techniques for applying the silicon to the setter thatinclude, but are not limited to, doctor blading, plasma-arc spraying andtape casting.

The purity level necessary in the sheet silicon is determined by therequirements of the specific application of the sheet. Whereas theemployment of low-processed metallurgical grade silicon is not adequatefor the efficient operation of a solar cell device, utilization ofhighly processed semiconductor grade silicon is not necessary. Inpractice, for direct solar cell applications the preferred process canbe executed with off-grade semiconductor grade silicon.

The addition of a separate constituent in, or with, the granular siliconmay be employed to effect the optical bandgap of the sheet. Additions ofcarbon, and in particular germanium, can increase (carbon) or reduce(germanium) the optical bandgap. Such changes in the optical bandgap ofthe sheet material are desirable depending on the spectral output of theincident radiation being employed with a solar cell design. In the caseof germanium combinations of silicon and germanium can be used whereeither can be from 0 to 100% (by mass).

The addition of a separate constituent in, or with, the granular siliconmay be employed to effect an electrical resistivity in the range of 0.1to 10 ohm-cm in the sheet material. Typically, for p-type conductivityin the sheet material the preferred elements are boron, aluminum, orindium. As an example of the preferred embodiment, the addition ofpowdered boron silicide followed by mechanical mixing of the granularsilicon provides for the accomplishment of the required p-typeresistivity in the subsequently grown silicon sheet.

The properly doped p-type granular silicon 200 is uniformly layered onthe coated setter 100. For example, in a preferred embodiment thisprocess can be effectively accomplished by using a doctor blade. Thespacing between the edge of the doctor blade and the setter surfaceneeds to be at least two times the dimension of the largest particle inthe granular silicon size distribution. Furthermore, the thickness ofthe final silicon sheet 600 can be the dimension of the largest particlein the granular size distribution.

The silicon-coated setter is transported into an environmental chamberwith an argon or nitrogen overpressure. In a preferred embodiment amixture of argon and hydrogen gas is employed to effectively limit theamount of silicon oxide that is formed during the growth process. Thepercent of hydrogen employed is determined by the water vapor content inthe chamber. The ratio of hydrogen to water vapor controls the magnitudeof silicon oxide formation. Preferably 5-100% by volume hydrogen gas isused to reduce the silicon oxide. The chamber may include a pre-heatzone employed to raise the temperature to 1100° to 1400° C., which incombination with the hydrogen has the effect of reducing the nativeoxide of silicon that exists on the granular silicon. In anotherpreferred embodiment a combination of nitrogen and argon (othernon-reacting gases such as helium, neon and krypton will also work) inthe environmental chamber is employed where either can be from 0 to 100%(by volume).

After the granular silicon 200 has been pre-heated it is then broughtinto a thermal zone 300 where the top portion of the granular siliconlayer 200 is melted. The depth of the granular silicon that is melteddepends on the intensity of the input energy from thermal zone 300, thethickness of the granular silicon layer, the linear speed of thegranular silicon coated setter through thermal zone 300, and the detailsof heat transfer between the granular silicon 200 and the setter 100.Between 25 and 90% (and preferably between 50% and 90%) of the granularsilicon depth is melted, primarily from the top. The material at thebottom of the granular layer is partially melted by liquid siliconpenetrating from the molten silicon layer above. This partially meltedlayer of silicon forms a net 220. The net 220 is responsible for twoprocess features. First, because it is wetted by the molten siliconabove, this layer stabilizes the melt and growth zones by defeating thesurface tension of the molten silicon over-layer. This allows theproduction of wide sheets, with smooth surfaces. Sheets widths of up to38 cm have been manufactured. Second, this layer can serve as a plane tonucleate subsequent growth (as described in U.S. Pat. No. 5,496,416).

FIG. 2 is a perspective view showing the sequence of nucleation andgrowth of the silicon sheet. A thin-film capping layer is formed on thetop surface of the liquid silicon 301 while it is in the melt zone 300.The role of the capping layer (nucleation layer 305 in FIG. 2) is toeffect the mechanism of heterogeneous nucleation. In this process (seeChalmers, Principles of Solidification, John Wiley & Sons, New York,1964) nucleation is controlled by the formation of nuclei of criticalsize catalyzed by a suitable surface in contact with the liquid. The“nucleation catalyst” or “nucleant” may be either a solid particlesuspended in the liquid, a liquid containing surface, or a solid film,such as an oxide or nitride. In this invention the “nucleant” may beapplied as a coating to the granular silicon before the introductioninto the environmental chamber or applied in situ.

In the grain growth process described by the present invention the rateof grain growth is determined by the details of heat extraction from themelt, and the grain size is determined by the nucleation density. Byemploying a nucleation layer 305 the nucleation occurs in a preferredmanner at the nucleation-layer/molten-silicon interface, and thenucleation density is actually reduced compared to a free molten liquidsurface. This allows for the achievement of controlled growth andincreased grain sizes in the manufactured sheet.

One method to effect a “nucleant” layer is to apply it to the granularsilicon prior to introduction into the environmental chamber. Thecoating materials in this manner to effect nucleation of the silicongrowth include, but are not limited to, the carbides, nitrides, oxidesand oxynitrides of silicon, the oxides and nitrides of boron, andaluminum oxide. The selected materials were chosen based on the need tomaintain required purity requirements of the resultant silicon sheet.

In another method the coating 305 may be formed in-situ on the freesurface at the top of the liquid 301 in the melt zone 300. The coatingmaterials employed on the free surface of the liquid 301 to effectnucleation of the silicon growth include, but are not limited to, thecarbides, nitrides, oxides and oxynitrides of silicon, the oxides andnitrides of boron, and aluminum oxide. The selected materials werechosen based on the need to maintain required purity requirements of theresultant silicon sheet. In a preferred embodiment the carbides,nitrides, oxides and oxynitride employed as coatings 305 on the freesurface of the melt 300 may be formed by the utilization of carbon,oxygen, and/or nitrogen containing gases as the process gas in theenvironmental chamber.

In the preferred embodiment the nucleation layer 305 is formed by thereaction of nitrogen in the process gas with the free liquid siliconsurface 301. For example, an effective nucleation layer is formed in acombination of nitrogen and argon (combined total 100%) when thenitrogen is 10% or greater (by volume). The reaction of the nitrogen gasand the free surface of the liquid silicon 301 forms a layer of nitridedsilicon. It is a further embodiment of the present invention that asmall amount of oxygen gas (10 to 1000 ppm by volume) can be added tothe nitrogen-argon combination to improve the nucleation properties ofthe nucleation layer 305.

After leaving the melt creation zone 300 of the thermal profile, themelt pool 301 with a nucleation layer 305 and the partially meltedsilicon net 220 moves into the nucleation and growth zone. FIG. 2 alsoillustrates the process occurring in the nucleation and growth zone 400.FIG. 2 indicates schematically the means of controlling the process ofnucleation and growth in zone 400 by managing the application andremoval of heat. This is depicted in the figure by Heat Management fromthe top 450, and Heat Management from the bottom 460.

Generally, nucleation and growth proceed as follows. A series ofpreferred nucleation sites 405 are formed in zone 400 at the interfacebetween the liquid silicon 301 and the nucleation layer 305. The detailsof this formation process are effected by the means of heat managementfrom the top 450 and the bottom 460. After formation of the preferrednucleation sites 405, grain growth 410 on these sites is effected bymodifying the heat management 450 and 460. The direction 470 of thegrain growth front is approximately perpendicular to the plane of thesetter, and perpendicular to the direction 610 in which the sheet isbeing pulled. The length of the growth zone along the direction ofsetter motion is from 10 centimeters to a maximum related to the sheetthickness, and the magnitude of the sheet pulling velocity and the graingrowth velocity. The length of the growth zone is determined bycontrolling the rate of loss of heat (and therefore growth rate)attending the solidification process. As a consequence of the growthprocess, the grains that are grown are columnar in nature. Typically,individual columnar grains 605 in the resulting sheet 600 have theiraxial direction extend from the top surface to the bottom, and are atleast as wide as they are high. Sheet thicknesses in the range of 400 to800 microns can be achieved at sheet pulling speeds in excess of 120cm/min.

After leaving the nucleation and growth zone 400 of the thermal profile,the sheet 600 moves into the annealing zone 500 of the thermal profile.In this zone the grown sheet, still at approximately 1400° C., issubjected to a near temperature gradient along the direction of settermotion. The linear temperature profile eliminates buckling and crackingof the as grown sheet, and minimizes the generation of dislocations. Thegrown sheet may have a thickness between 50 microns and 2 mm. Thethickness of the grown sheet is in the range of 350 to 1000 microns inthe preferred process. Because the thickness of the final grown sheet600 is determined by the precise application of granular silicon 200 tothe setter 100, exceptional sheet thickness control and processstability are achieved in comparison to sheet technologies pulled from amelt, where thickness is controlled by the melt meniscus. After cooldown, the sheet is removed from the setter, and appropriately sized bysawing or scribing, for fabrication into solar cells. The setter isreused for making further columnar-grained polycrystalline sheets.

FIG. 3 depicts the process steps that can be employed by the inventionto achieve the silicon sheet at high sheet pulling velocities. There areseveral nucleation and growth regimes that have been successfullyemployed. Below are several examples derived from FIG. 3.

The properties of the sheet material fabricated with the above processare quite amenable to the fabrication of efficient solar cells. Thisprocess generates material that has unique properties of size andcharacter. Although the grains are columnar, and have average sizes inthe range of 0.002 to 1.000 cm in extent, solar cells fabricated onmaterial in the range of 0.01 to 0.10 cm may achieve voltages in excessof 560 mV, and fill factors in excess of 0.72. The achievement of thesevalues on such small grained material indicate that this material is notbeing limited by recombination at grain boundaries as had beenpreviously predicted by Ghosh. Previously, columnar grains that extendfrom surface to surface of the sheet were dismissed as being ineffectivesince columnar grains were always small, and small grains were thoughtnot to work. The process herein described achieves columnar grains thatyield material with relatively benign grain boundaries with the resultthat efficient, low-cost solar cells can be manufactured.

It is a unique feature of the top-down grain growth process describedabove that the device-active region at the top of the sheet iscrystallized from the silicon melt while the bottom of the sheet next tothe setter is still solid, thus minimizing any contamination of the topof the sheet by the setter. It is a further advantage of the top-downgrain growth process that purification of the device-active region iseffected by fractional solidification (see: Zief and Wilcox, FractionalSolidification, Marcel Dekker, New York, 1967). By this method the mostpure grown material is at the top of the sheet where the initial graingrowth occurs. The subsequent grain growth process has the effect ofsweeping impurities to the bottom of the solidified sheet away from thedevice-active region (similar to zone refining).

The process herein described can be carried out in a continuous manner,resulting in continuous sheets that can be appropriately sized using anin line scribe or a saw. Impurity content in the melt and grown sheetquickly reaches steady-state; it does not increase during continuousprocessing. Since all embodiments include application of granularsilicon to the setter, and since material enters the melt creation zonein this form, melt replenishment is not a problem, unlike sheettechnologies pulled from a melt pool. After being properly sized, thesheets function as a substrate by having the remaining layers formedthereon to produce solar cells. Where the remaining layers are ofsilicon (or an alloy to tailor the optical bandgap, such as carbon orgermanium), a complete solar cell results. This sheet material can alsobe the substrate for other solar cell fabrication processes includingthose employing cadmium telluride and copper-indium selenide. Thus anadvantage of the invention is that it lends itself to mass production.The invention results in a free standing grown continuous sheet ofsilicon which could then be cut to individual sizes in accordance withits end use.

The present invention anticipates the forward integration of the siliconsheet product in subsequent processing into solar cells and modules. Itis expected that the solar cell fabrication steps, including, but notlimited to, surface preparation, junction formation, electrical contactsand anti-reflection coatings, can all be accomplished with the productin its sheet form. Such continuous processing will have the advantage ofsignificantly reducing manufacturing costs.

EXAMPLE 1

The mechanism of spontaneous nucleation (see FIG. 3) is operative whenthe liquid silicon 301 has become super-cooled, and in the absence ofany uniform extraction of heat in the nucleation and growth zone 400,results in a solid-liquid interface that is inherently unstable. Thisinstability leads to dendritic growth which is typically equiaxed ingeometry. Such grain growth may or may not extend from one sheet surfaceto the other.

EXAMPLE 2

In another variation of the process (see FIG. 3) the silicon net 220serves as the source of nucleation of grain growth from the liquidsilicon 301. In this case the heat is most effectively extracted fromthe bottom 460. In order to reduce the rate of grain growth it may bedesirable to apply some heat from the top 450. Heat extraction from thebottom can be effected by means of radiation plates. Heat addition atthe top is effected by means of planar heaters. For example, to effect agrain growth rate of 0.15 cm/min for a 700 micrometer thick sheet movingat sheet speed of 100 cm/min requires a net heat removal rate from thebottom (i.e. heat extracted from bottom minus heat added to top) ofapproximately 10 watts/cm2, and results in a nucleation and growth zonelength of 46 cm. In this process regime the solid-liquid interface isstable leading to the growth of columnar grains that extend from the netat the bottom of the sheet up to the free surface of the sheet. Thisexample has been described in U.S. Pat. No. 5,496,416.

EXAMPLE 3

In another variation of the process (see FIG. 3—formation of, and growthfrom, a nucleation layer) granular silicon in the size range of 75 to350 micron is doctor-bladed onto a graphite setter coated by a moldrelease coating consisting of a mixture of the oxides and nitrides ofsilicon. The granular silicon and setter are then transported at acontinuous velocity through a graphite-based heating chamber under a100% nitrogen gas atmosphere bringing the silicon and setter to atemperature close to 1400° C. During this time the majority of siliconoxide that existed on the granular silicon surfaces is volatilized andsubsequently removed from the heated chamber. The silicon and settercontinues in the chamber to a zone where the silicon is melted,primarily from the top, leaving a silicon net at the bottom. During thistime a thin-film capping layer of silicon nitride is formed on the freesurface of the silicon. The thin-film of silicon nitride plays the roleof a nucleation layer in the next zone of the chamber where minimalexternal heat is applied and the nucleation sites are preferentiallycreated at the thin-film layer/molten silicon interface. The layercombination of setter, silicon net, liquid silicon and thin layer ofsilicon nitride with an array of nucleation sites continues into a zonewhere heat is preferentially removed from the top 450. In this case, thegrains grow down from the nucleation sites occurring at thethin-film/molten silicon interface and terminate at the silicon net atthe bottom. The grain growth rate is determined by the details of heatextraction 450. In this process regime the solid-liquid interface isstable leading to the growth of columnar grains that extend from thenucleation layer at the top of the sheet to the net at the bottom of thesheet.

EXAMPLE 4

Same as Example #3 except after an initial top-down grain growth fromthe nucleation layer through most of the molten thickness, the sheetmoves into a subsequent zone that adds heat from the bottom 460, suchthat the net layer and some portion of the previously top-down growngrains are melted. The sheet then continues to a zone where the heat isagain extracted from the top 450. The growth now resumes from thesolid-liquid interface of the initially grown grains and continues allthe way through the sheet to the bottom of the sheet. In this processregime the solid-liquid interface is stable and results in the growth ofcolumnar grains that extend from the nucleation layer at the top of thesheet to the bottom of the sheet. This process also has the capabilityof sweeping out impurities to the back of the sheet by a process similarto zone refining.

These examples are for illustrative purposes and are not intended torepresent all the processes anticipated by the present invention.

What is claimed is:
 1. A sheet of silicon having a pair of freesurfaces, columnar grains extending axially through the sheet from onefree surface to the other free surface, said sheet having an electricalresistivity in the range of 0.1 to 10 ohm-cm, and said surfaces beingfree of any outer integral residual net.
 2. The sheet of claim 1 whereinthe sheet includes at least some amount of nitrogen.
 3. The sheet ofclaim 2 wherein said sheet functions as a substrate, and the sheet beingin combination with photovoltaic layers to comprise a solar cell.
 4. Thesheet of claim 1 wherein said sheet functions as a substrate, and thesheet being in combination with photovoltaic layers to comprise a solarcell.